FOUS30 KWBC 100924
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
424 AM EST Sun Nov 10 2024

Day 1
Valid 12Z Sun Nov 10 2024 - 12Z Mon Nov 11 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS CENTRAL AND
SOUTHWEST LOUISIANA...

Southwest and Central Louisiana...
Moisture and instability from the Gulf gets advected from the
vicinity of Raphael northwest to north across portions of the
Central Gulf Coast through the Tennessee Valley. A slowly
advancing convective complex early this morning across southwest LA
is drifting eastward, showing hourly rain totals to 3" and
isolated totals to 13" thus far over portions of Allen Parish.
There is concern that this complex could hold together past 12z,
which is part of the reason for the upgrade to a Slight Risk, after
coordinating with the LCH/Lake Charles LA forecast office.
Additional activity could form behind it closer to a surface
boundary in TX (could be forming at the present time) which is
expected to advect eastward during daytime heating. Moderate Risk
level impacts cannot be ruled out should such thunderstorm activity
persist over recently saturated soils into this afternoon.

Southeast Louisiana...
The guidance broadly remains unclear as to whether heavy rainfall
gets too far ashore southeast LA while mesoscale guidance shows the
complex to the west fizzling as it moves into the region.
Precipitable water values of 1.5-2" Sunday morning across southeast
LA advect offshore with time, which should limit heavy rainfall
concerns. Still, hourly rain totals to 2" and local totals to 4"
are possible on an isolated basis should storms backbuild or train.

Interior Southeast/Mid-South/Tennessee Valley...
The forward progress of the front should keep any excessive
rainfall concerns minimal in areas a bit north of the Gulf Coast
-- some of the risk area from eastern AR to the east-northeast just
south of the OH river accounts for recent rains/some soil
saturation. In this more northern portion of the Marginal Risk
area, hourly totals up to ~1" with local totals to 2" appear to be
the maximum potential. A decent amount of the area within the
Marginal Risk would find the forecasted rainfall welcome.

Changes to continuity were to shave off some of the northeast
portion of the Marginal Risk area, using 00z HREF probabilities of
0.5"+ as a guide, and include some of far southeast Texas to
account for the limited QPF signal there as soils are a bit more
sensitive after the past 24-36 hours of heavy rainfall. 

Roth


Day 2
Valid 12Z Mon Nov 11 2024 - 12Z Tue Nov 12 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Roth


Day 3
Valid 12Z Tue Nov 12 2024 - 12Z Wed Nov 13 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN AND NEAR
WESTERN WASHINGTON...

Precipitable water values of 0.75-1" advect in as a cold front
approaches, perhaps moves ashore, the Washington and northern
Oregon coast Wednesday morning. Inflow from the Pacific rises to
60 kts at 850 hPa out of the south-southwest with limited
instability available near and behind the front. The ingredients
appear to be there for hourly rain totals of 0.5"+, which would be
most problematic in burn scars. Over the full day, local amounts
of 3-4" are advertised for much of the guidance, which would be
most problematic along the Skokomish river in the Olympic
Peninsula. Enough was there in the guidance for the inherited
Marginal Risk area to remain from continuity; simplified its shape.

Roth

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